Engines may be configured with an exhaust gas heat recovery (EGHR) system for recovering heat from exhaust gas. During lower engine temperature and/or during vehicle cabin heating demands, exhaust may be routed via the EGHR system and exhaust heat may be recovered by coolant flowing through a heat exchanger of the EGHR system. Coolant with the recovered exhaust heat may be circulated via the engine and/or the heater core of an on-board heating, ventilation, and air conditioning (HVAC) system, and exhaust heat may be utilized for providing heat to the engine and also to warm the vehicle cabin, thereby improving engine, and fuel efficiency. A diagnostic procedure may need to be periodically or opportunistically carried out to monitor different components of the EGHR system including the coolant temperature sensors housed in coolant lines fluidically coupled to the heat exchanger of the EGHR system.
Various approaches are provided for diagnostics of engine coolant temperature sensors. In one example, as shown in U.S. Pat. No. 6,848,434, Li et al. discloses a method for diagnosing a coolant temperature sensor coupled to an engine coolant system. A coolant temperature may be modeled based on each of an energy flow between engine and coolant, an energy flow between coolant and air, and an energy flow from coolant to radiator. Upon indication of engine warm up based on the modeled coolant temperature, diagnostics of the coolant sensor may be carried out based on a comparison of a coolant temperature estimated via the coolant temperature sensor and a pre-determined regulated temperature.
However, the inventors herein have recognized potential issues with the above approach. As one example, in embodiments having an EGHR system, multiple coolant temperature sensors may be housed in the coolant lines coupled to the EGHR system and rationality of each coolant temperature sensor needs to be monitored independently. The coolant temperature at each location of the EGHR system may be different at distinct modes of operation of the EGHR system and it may not be possible to use a single model for computing coolant temperature at each location in the EGHR system during operation of the EGHR system in each operating mode. Further, by modeling coolant temperature solely based on energy flow between each of the engine, coolant, air, and radiator, it may not be possible to quantify energy loss during heat transfer between the aforementioned components, thereby reducing the accuracy of the temperature model.
The inventors herein have identified an approach by which the issues described above may be at least partly addressed. One example method comprises, during exhaust flow from a vehicle engine across a heat exchanger having a coolant flowing there through, in response to a higher than threshold difference between a measured coolant temperature and a modeled coolant temperature which is based on heat transfer between a heat loss source and a vehicle cabin, indicating degradation of a first and/or a second coolant temperature sensor respectively coupled upstream and downstream of the heat exchanger. In this way, by using separate temperature models for computing coolant temperature at distinct locations in the EGHR system and comparing measured coolant temperature to modeled coolant temperature, degradation of one or more coolant temperature sensors may be independently detected.
In one example, an engine system may be configured with an exhaust gas heat recovery (EGHR) system including a heat exchanger. The heat exchanger may be positioned in an exhaust bypass passage, disposed parallel to a main exhaust passage, and a diverter valve coupled to the main exhaust passage may be used to enable exhaust to be diverted into the bypass passage or directed through the main passage into the tailpipe. Based on engine heating and/or vehicle cabin heating demands, the EGHR system may be operated in a plurality of modes by adjusting a position of the diverter valve. As an example, during higher engine and/or cabin heating demands, the diverter valve may be actuated to a first position (first mode of operation of EGHR system) to flow exhaust to the tailpipe via the heat exchanger and once the engine temperature has increased above a threshold and vehicle cabin heating is no longer desired, the diverter valve may be actuated to a second position (second mode of operation of EGHR system) to flow exhaust directly to the tailpipe bypassing the heat exchanger. During exhaust flow via the heat exchanger, engine coolant may be routed via the heat exchanger, wherein heat from the exhaust may be transferred to the coolant. The coolant with the exhaust heat may then be routed via the engine and the heater core of the vehicle HVAC system wherein the exhaust heat may be used to increase engine and/or vehicle cabin temperature. A first coolant temperature sensor may be coupled to a first coolant line entering the heat exchanger (upstream of the heat exchanger) and a second coolant temperature sensor may be coupled to a second coolant line exiting the heat exchanger (downstream of the heat exchanger). During operation of the EGHR system in the first mode when exhaust is routed via heat exchanger, coolant temperature upstream of the heat exchanger may be modeled using two distinct modeling approaches. Each of the two modeling approaches for computing coolant temperature upstream of the heat exchanger may be based on heat loss during heat transfer from the heater core and coolant lines to the vehicle cabin. The modeled coolant temperature upstream of the heat exchanger may be calibrated and optimized over a plurality of HVAC system operating conditions upstream of the heat exchanger. Coolant temperature downstream of the heat exchanger may be also modeled using two distinct modeling approaches. Each of the two modeling approaches for computing coolant temperature downstream of the heat exchanger may be based on heat loss during heat transfer from the exhaust gas to the heat exchanger and the modeled coolant temperature upstream of the heat exchanger. The optimized modeled coolant temperature upstream of the heat exchanger may be compared to a measured coolant temperature and in response to a higher than threshold difference between the modeled temperature and the measured temperature, degradation of the first coolant temperature may be detected. Similarly, the modeled coolant temperature downstream of the heat exchanger may be compared to a measured coolant temperature and in response to a higher than threshold difference between the modeled temperature and the measured temperature, degradation of the second coolant temperature may be detected. In addition to diagnostics of the coolant temperature sensors, during operation of the EGHR system in the exhaust heat, diagnostics of the diverter valve and the heat exchanger may also be carried out.
In this way, by using distinct mathematical approaches to model coolant temperature upstream and downstream of a heat exchanger of the EGHR system, degradation of a first coolant temperature sensor upstream of the heat exchanger may be differentiated from degradation of a second coolant temperature sensor downstream of the heat exchanger and appropriate mitigating actions may be performed. By opportunistically carrying out the diagnostics of the coolant temperature sensors, the diverter valve, and the heat exchanger during specific modes of operation of the EGHR system, the possibility of false detection of degradation of the one or more components of the EGHR system may be decreased. The technical effect of including heat loss between one or more vehicle components including the heater core, the coolant lines, the vehicle cabin, exhaust gas, and the heat exchangers in computation of the modeled temperature is that accuracy of the modeled temperature upstream and downstream of the heat exchanger may be improved. By enabling diagnostics of the coolant temperature sensors to be carried out reliably and accurately, propensity of coolant overheating may be reduced and robustness of the HVAC system may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.